1. Field of the Invention
The present invention relates to a high frequency substrate, and more specifically to a high frequency substrate capable of increasing the signal transmission speed of the high-frequency signal transmission line and maintaining the power plane and the ground plane in a stable voltage status.
2. Description of the Related Art
While wireless network and satellite communication are gaining rapid growth, electronic products tend to have the design of high speed, high frequency and high capacity. Consequently, the present electronic products need to have the support of a high frequency substrate in order that a high frequency and high speed performance can be achieved.
Please refer to FIG. 1, tbe cross-sectional view of a conventional high frequency substrate. In FIG. 1, the high frequency substrate 100 comprises a power plane 102, a ground plane 106, a high-frequency signal transmission line 110 and dielectric layers 104 and 108. The dielectric layer 104 is formed on the power plane 102; the ground plane 106 is formed on the dielectric layer 104. The dielectric layer 108 is formed on the ground plane 106 while the high-frequency signal transmission line 110 is deposited on the dielectije layer 108. The dielectric constant of the air outside the high-frequency signal transmission line 110 is 1 while the dielectric constant of the dielectric layers 104 and 108 is εr2 wherein the value of εr being greater than 1, say 4 for instance. Besides, the dielectric layers 104 and 108 are normally dense structures.
Suppose that the signal transmission speed in the high-frequency signal transmission line 110 is Vp, then Vp=C/(εeff)1/2, wherein C is the speed of the light while εeff is the effective dielectric constant. That is to say, the signal transmission speed Vp is inversely proportional to the square root of the effective dielectric constant εeff. It is noteworthy that εeff varies with the intensity of signal transmission frequency. In other words, an electromagnetic field will be generated during transmission. The dielectric constant for the medium of the high electromagnetic area can be regarded as εeff.
Unlike in the case of low-frequency operation where the electromagnetic field generated during signal transmission concentrates in the air outside the high-frequency signal transmission line 110, the electromagnetic field generated during signal transmission almost entirely concentrates in the dielectric layer 108 because the high-frequency signal transmission line 110 normally has a high frequency of signal transmission. So the effective dielectric constant εeff approximatly equals to the dielectric constant εr of the dielectric layer 108. For instance, the value of εeff equals 4. Hence, the signal transmission speed Vp will slow down, which increases the energy loss of signals and in turn greatly affects signal transmission quality.
Moreover, the voltages of the power plane 102 and the ground plane 106 are interfered with one another due to the low diclectric constant of the dielectric layer 104; for example, εr is 4. The values of voltage in the power plane 102 and the ground plane 106 vary; the power plane 102 and the ground plane 106 are unable to be maintained in a stable voltage status so that the high frequency substrate 100 is subjected to huge impacts and unable to keep in well progress.